1. Field of the Invention
The present invention relates to a lead frame for a semiconductor package and a semiconductor package in which the semiconductor device is mounted on a lead frame and the outside thereof, particularly, the top surface of the semiconductor device, is encapsulated with molding compound.
2. Description of the Related Art
In recent years, it has been required to miniaturize and shape a semiconductor product mounted on a substrate so as to be thinner, as the packaging of the semiconductor is made more dense. It has been required for LSI to reduce the number of chips by improving integration level and to miniaturize and make a package lighter. The popularization of so-called CSP (Chip Size Package) is rapidly advancing. Particularly, in the development of thin semiconductor products with lead frames, the semiconductor package of the single side encapsulation type has been developed in which a semiconductor device is mounted on a lead frame and the surface of the semiconductor device is encapsulated with molding compound.
FIG. 1 is a sectional view of one example of a semiconductor package. FIG. 2 is a plan view thereof seen through molding compound. The semiconductor package shown in FIGS. 1 and 2 is comprised of a lead frame 1, a semiconductor device 4 mounted on die-pad 3 supported with suspending leads 2 of lead frame 1, metallic thin wires 6 electrically connecting electrodes provided on the top face of the semiconductor device 4 with terminals of lead frame 1, and molding compound 7 for encapsulating the outside region of semiconductor device 4 including the upper side of semiconductor device 4 and the lower side of die-pad 3. The semiconductor package is of the so-called non-lead type of semiconductor package in which outer leads do not project from the semiconductor package and the inner leads and outer leads are integrated into terminals 5, wherein in lead frame 1, suspending leads 2 are directed upward in such a manner that die-pad 3 is positioned higher than terminals 5. Since such a step is provided between die-pad 3 and terminals 5, molding compound 7 can be inserted into the lower side of die-pad 3.
Since a semiconductor device is miniature, a matrix type frame is mainly used for the above-mentioned non-lead type of semiconductor package, in which plural semiconductor devices are arranged in a direction of a width of the matrix type frame. Further, recently, due to a demand for lower costs, it is thought to switch over from a frame of the individually molded type shown in FIG. 3 to a frame of the collectively molded type shown in FIG. 4.
In the frame of the individually molded type, as shown in FIG. 3(A), individual molding cavities C of small size are provided separately within a frame F. After molding, individual semiconductor packages are stamped out so that semiconductor packages S shown in FIG. 3(B) are obtained. Namely, semiconductor devices are mounted on die-pads of lead frames through silver paste and others, and wire bonding is carried out. Thereafter, respective semiconductor devices are individually molded with molding compound and the molded semiconductor devices are stamped out to form individual semiconductor packages.
In the frame of the collectively molded type, as shown in FIG. 4(A), some molding cavities C of large size are provided within a frame F. Multiple semiconductor devices are arranged in a matrix within each molding cavity C, and are respectively and collectively molded with molding compound. Thereafter, the collectively molded semiconductor devices are cut at grid-leads L by means of a dicing saw so that a semiconductor package S shown in FIG. 4(B) is obtained. That is, semiconductor devices are mounted on die-pads of lead frames through silver pastes and others and wire bonding is carried out. Thereafter, plural semiconductor devices are collectively molded with molding compound to a given cavity size, and then the collectively molded semiconductor devices are cut to form individual semiconductor packages by dicing.
Generally, in the above-mentioned semiconductor package, in order to facilitate wire bonding in the process of forming the semiconductor package, noble metal plating such as silver plating is previously given to parts of terminals of the lead frame to be wire-bonded. As to the areas of noble plating, in a semiconductor package of such a type that outer leads project from the semiconductor package, there are many standards that the plating areas of noble metal are restricted between a point 0.3 mm away from the ends of the inner leads and the inside of the dam bar. According to the standards, judgment on whether the plating areas of noble metal are good or defective can be easily made by visual observation. On the other hand, in case of the above-mentioned non-lead type of semiconductor package, plating areas are small, wherein the accuracy of plating areas is assured with values measured in the random sampling. Therefore, increased labor is required for the measurement of the size of the plating areas, which increases costs.
Further, the non lead type of semiconductor packages are separated from each other by punching or dicing, wherein there was a problem that peeling is produced between the molding compound and the terminals of the lead frame, since stress is applied to the terminals of the lead frame in separating the semiconductor packages from each other. Further, there was a problem in that the reliability of assembly after mounting the semiconductor package on a printed circuit board is poor.
Further, in the case of giving noble metal plating to parts of the terminals to be wire-bonded according to a method of plating using masking means, there was a problem that leakage of the least amount of plating solution through a mask of the masking means always occurs.